It has long been known to provide work vehicles with endless track assemblies, or tracks (sometimes referred to as "crawler treads"), for ground support in order to provide increased traction and support, or flotation, upon moist or loosely packed soils. Due in part to their larger footprints which compact agricultural field soils less than do the tires of wheels of typical agricultural tractors, tracked vehicles have in recent years become increasingly popular in crop farming as well, particularly in connection with larger, heavier and more powerful tractors. Track assemblies, including undercarriage such as idler and bogie wheels, pivot brackets and other supporting structure may be referred to as track suspensions.
Maneuverability of a tractor with a connected implement is of importance, especially at the headlands of a field (i.e., at the ends of crop rows) where a tractor with implement must be turned 180 degrees. Larger wheeled tractors, which are provided with rigid frames having larger turning radii if provided with conventional front wheel steering systems, can require excessive maneuvering and/or width of headland. Their frames are therefore often hinged, or articulated, and they are steered by changing the angle of a front frame with respect to a rear frame, typically by use of a hydraulic cylinder bracketing a pivot joining the front and rear frames. This is an expensive configuration of construction, however, and is generally used only on very large tractors.
Larger tracked tractors utilizing articulated frames are produced in the QUADTRAC.TM. series of models by Case Corporation, assignee of the present invention. These utilize four endless track assemblies, one each generally located near the front and rear ends of the right and left sides of the tractor. Other tracked tractors generally utilize rigid frames with one endless track on the left side and another on the right side, and are steered by changing speed of either endless track with respect to the other. Such tractors can turn in extremely tight circles, but, in the hands of an inexperienced or inattentive operator, can easily and quickly be inadvertently jackknifed with an implement and damage the implement. Moreover, they provide a harsh ride and generally lack the directional stability desired for ease in making or following long, straight and parallel crop rows.
Because various types of crops often require differing row spacings for efficient utilization of a field, tractors are preferably provided with adjustable track width; i.e., the widths in front and rear elevation views between centerlines of tires or endless track assemblies. While this is relatively easily accomplished with wheeled tractors, it is generally a time-consuming and costly operation with a tracked tractor, requiring significant undercarriage disassembly.
It would be advantageous to provide for a work vehicle, such as an agricultural tractor, to offer the better ride and directional stability of a wheeled tractor while also offering the better traction and lesser soil compaction of a tracked tractor, without incurring the expense of an articulated configuration.
It would also be advantageous to provide for such a work vehicle to include endless track drives which are compact and which are relatively easily repositioned to change vehicle track width.